The present invention is directed to a bubble memory constructed of a major loop-minor loop field access design having a number of defective minor loops and more particularly to a bubble memory chip having on-chip firmware providing redundancy information without seriously limiting the amount of useful data capable of being stored in the memory. Magnetic domain devices have been increasing in use due to demand for a high-speed large capacity memory device. These magnetic domain devices are generally planar in configuration and are constructed of material which has magnetically easy directions which are essentially perpendicular to the plane of the structure. Magnetic properties such as magnetization anisotropy, coercivity and mobility associated with magnetic domain devices allow such device to be maintained magnetically saturated with magnetism in a direction out of the plane and that small localized signal domain regions of magnetic polarization aligned opposite to the general polarization direction may be supported. Such localized regions which are generally cylindrical in configuration repressent binary memory bits. A magnetic domain can be manipulated by programming currents through a pattern of conductors positioned adjacent the magnetic material or by varying the surrounding magnetic field. Since the magnetic bubbles can be propagated, erased, replicated and manipulated to form data processing operations and their presence and absence detected, these bubbles may be utilized as memory devices.
Many circular organizations of operable magnetic domains have been disclosed in the prior art. One example is that of the major-minor loop memory organization disclosed in U.S. Pat. No. 3,618,054. The major-minor loop memory organization as well as its implementation and operation is well-known in the art. The major-minor loop organization includes a closed major loop which typically is established by an arrangement of T-bar permalloy circuits on, for example, an orthoferrite platelet. The magnetic domains are propagated around the loop by in-plane rotating magnetic field action. The major loop is generally elongated to permit a number of minor loops to be aligned alongside and perpendicular to it. Two-way transfer gates permit the transfer of magnetic domains from the minor loop to the major loop and from the major loop to a minor loop. Further access to the major loop is achieved by a detect and read connection thereto and by a separate write connection. This type of organization permits a synchronized domain pattern with propagation in the loops synchronous with the rotation of the in-plane field. That is, parallel transfer of data domains from a plurality of minor loops may be made simultaneously to the major loop. Moreover, a plurality of data chips, each with a major loop and a plurality of associated minor loops, may be treated together.
As is well-known in the art, all of the minor loops in the chip, upon command, transfer in parallel the bubble from their corresponding positions to the major loop. The bubbles are then serially detected as they are propagated passt the read position. New data may also be inserted at a write position for parallel transfer back into the minor loop at an appropriate later time, which in most cases occurs when the minor loop's magnetic domain propagation aligns the data for transfer.
Another structural organization of operable magnetic domains is the block replicate organization, also well-known in the art. The block replicate organization includes open-ended major propagation paths which may be established by an arrangement of T-bar permalloy circuits on, for example, a rare earth orthoferrite platelet. These major propagation paths are aligned adjacent to a plurality of minor loops. Data is written into the minor loops from a major propagation path by way of a swap transfer gate. Old data is transferred into the major propagation path by a swap signal received from a control chip as ultimately annihilated. A subsequent swap signal transfers new data into the minor loops where it becomes non-volatile. To read data out of the minor loops in a block replicate organization it is necessary to read out the magnetic domains onto separate major propagation paths. Replicate gates between the minor loops and the major propagation paths allow the stored data to remain within the minor loops with the data that is read out onto the major propagation path being a replicated version of stored data. The major distinction between a block replicate organization and a major-minor loop organization is that the data stored within the minor loops remain in the minor loops during the read operation mode in a block replicate, while the stored data is transferred completely out onto a major propagation path before replication to a user system in a major-minor loop organization. Also, since it is not physically possible to locate the minor loops so as to take advantage of all locations on the major propagation paths, the rate of bubble movement within the restrictive minor loops is greater than that possible at the detector. In order to overcome this physical disability, the major propagation paths in the block replicate organization and the output for the reading of the minor loops are merged, making one major propagation path shorter by one position as compared to another major propagation path allowing a merger of the two paths with one path complementing the void present in the other path. The result of the merger is to double the data rate out of the major loop to the detector making it equal to the rate within the minor loops.
In both the block replicate organization and the major-minor loop organization, unless special provision is made, every loop and every chip of the system must be perfect for the system to perform satisfactorily. Since chips contain entire groupings of registers, a defect in one of the minor loops would require discarding the entire chip. Various techniques have been proposed in the art for permitting the use of a magnetic domain chip even though one or more of its minor loops may be defective. Examples of such techniques may be found in U.S. Pat. Nos. 3,908,810 and 4,090,251. As disclosed in these patents, the technique used includes the use of a non-volatile semiconductor memory, such as a programmable read-only memory, to store data identifying the relative position of defective minor loops to each other or the storing of a redundancy loop pattern in the first page of the plurality of minor loops which takes the form of a series of magnetic domains with the presence of a magnetic domain designating only those loops that are capable of propagating magnetic domains. A page of information is defined to be the contents of a slice of one in the same bit position in all minor loops. A second redundancy pattern is stored in the second page of the plurality of minor loops in the form of loop numbers designating those minor loops that are capable of propagating magnetic domains while a third redundancy pattern stored in the third page of the plurality of minor loops in a form identical with that of the first page. Reading the redundancy patterns in the first three pages of the plurality of minor loops enables those minor loops which are defective to be found. The use of this type of structure is either costly or reduces the amount of storage area that is available for use by the processing system. Accordingly, it is an object of the present invention to provide an improved system of using major-minor magnetic domain organization to permit the use of data chips having one or more faulty minor loops. It is another object of this invention to provide an improved major-minor loop magnetic domain memory system which overcomes the problems stated above, while of low cost construction but retaining a high storage capacity.